Emerging evidence suggests that elevated depressive symptoms predict stroke onset. However, it remains unclear whether this relationship is causal and, if so, whether the effects are modified by the duration or intensity of depressive symptoms. Furthermore, despite substantial research, we have not been able to establish the mechanisms linking depression and stroke. Addressing these research questions is critical to inform clinical management of depressive symptoms. It also is useful in elucidating the physiological or behavioral mechanisms mediating the link between depression and stroke to identify other opportunities for intervention. Previous research suggests several possible pathways via which depression or depressive symptoms could influence stroke, including health behaviors (e.g. smoking, physical activity) or dysregulation of physiologic processes (e.g. autonomic regulation of cardiac function, inflammatory responses). Many of these mechanisms would operate over a long term time-scale, with pathophysiology (e.g. atherosclerosis) accumulating over years. If only long-term mechanisms link depression and stroke, treatment of depressive symptoms would not be expected to reduce stroke risk immediately; benefits would instead develop over years of successful symptom management. However, if causal mechanisms exert their effects in the short term, interruption of some hypothesized mechanisms might allow nearly immediate benefits by reducing stroke risk after resolution of depressive symptoms. Furthermore, it remains unclear if antidepressant medication affects the risk of stroke. The research on health behaviors and inflammation response has not conclusively established whether these factors fully mediate the relationship between depression and stroke. While no prior research has assessed whether atrial fibrillation (AF) mediates the relationship between depression and stroke, a growing number of studies are examining the impact of mood on AF. As the most common cardiac arrhythmia and a well-known risk factor of stroke, we will examine AF's role as a possible mediator. We propose using data from two complementary longitudinal studies, the Health and Retirement Study (HRS) and the Cardiovascular Health Study (CHS), to examine two aims. The first aim of this proposal is to determine what characteristics (e.g. duration and severity) of depressive symptoms best predict first incidence of all stroke types among middle aged and elderly individuals in the United States. Our second aim is to examine atrial fibrillation as a partial mediator of the association between depressive symptoms and onset of ischemic stroke. The identification of characteristics of depressive symptoms that predict stroke and the underlying etiology can help health practitioners and identify patients at greater risk for stroke. Indentifying whether atrial fibrillation is a mediating factor in the relationship between depression and stroke is clinically important as it may inform treatment plans of at risk patients.